Optically-coupled data access arrangement and transhybrid

ABSTRACT

Isolation and interconnection of a telephone network of tip and ring lines and the analog ports of a modem are achieved with a transhybrid that incorporates optically-coupled isolation stages. Optical isolators connect to the tip and ring lines to provide optimal isolation and unidirectional transfer of signals.

This is a divisional of application Ser. No. 08/191,841 filed Feb. 4,1994.

FIELD OF THE INVENTION

This invention generally relates to telephone line communications.Specifically, the invention is directed to coupling devices fortelephone subscriber equipment known as data access arrangements orDAAs.

BACKGROUND OF THE INVENTION

Telephone signals are provided to subscribers (customers) through thepublic switched telephone network. The subscriber portion of the networkhas two wires known as tip and ring, which carry the ring signal as wellas the information being transferred. The bandwidth of the network isapproximately 300 Hz to 3.3 kHz. Necessarily, any terminal equipmentconnected to this network must meet certain specifications in order tofunction properly.

To connect subscriber equipment such as data modems, facsimile machines,(non-cellular) portable telephones, speaker telephones, and messageanswering machines to the analog public switched telephone network, onemust provide an interface or data access arrangement to bridge anyincompatibilities between the network and the subscriber equipment. Inaddition to complying with network protocols, since the subscriberequipment are four-wire devices with separate transmit and receivepairs, the interface must separate the analog signals on the networkinto discrete transmit and receive signals (and vice versa). Finally,the interface must electrically isolate the telephone network from thesubscriber equipment.

The interface must be transparent, so that from the vantage point of thecentral office, the line appears to be terminated in a conventionalanalog telephone set. Thus, the interface must simulate the D.C.continuity indicating the "off hook" condition when a call is receivedor placed. Typically, the separation function is performed by a circuitcontained within the interface, called a 2-to-4 wire transhybrid.

For applications such as small portable computers and data entrydevices, an interface having minimal volume and weight is ideal.However, the size cannot be arbitrarily reduced without impacting theperformance. For example, some interface methods use isolationtransformers to provide the required D.C. separation between the networkand the subscriber, which offer wide bandwidth and transparency.However, transformers suited for this application suffer from beingphysically large, heavy, and costly, and therefore represent a poorchoice for portable devices. If the transformer is eliminated from theinterface, a substitute providing the requisite performance is needed.

An ideal interface or data access arrangement should offer a flatfrequency response, constant group delay, extremely minimal amplitudeand frequency distortion, and reflect the proper line impedance. Itwould be desirable to provide an interface not requiring the traditionalline transformer but yet offers the same or a superior level ofperformance.

SUMMARY OF THE INVENTION

These and other objects are achieved by an interface that has atranshybrid circuit incorporating isolation. In addition to separatingthe transmit and receive signals, the transhybrid also provides D.C.isolation between the telephone network and the subscriber equipment.The embodiment disclosed here uses optical couplers to achieve theisolation. A group delay equalizer and a differential amplifier suppressthe propagation of the transmit signal to the receive output.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, as well as otherobjects and advantages thereof not enumerated herein, will becomeapparent upon consideration of the following detailed description andthe accompanying drawings, wherein:

FIG. 1 is a schematic block diagram of a data access arrangementcircuit;

FIG. 2 is a schematic diagram of an optically-coupled transhybrid: and

FIG. 3 is a schematic diagram of a data access arrangement circuithaving an optically-coupled transhybrid.

DETAILED DESCRIPTION

The block diagram of FIG. 1 illustrates the configuration of a dataaccess arrangement ("DAA") 10. The DAA 10 provides an interface betweenthe public telephone network 20 (originating at the central office),with tip and ring lines 22 and 24, and the subscriber's digital data bus30, in turn connected to subscriber equipment 32. The DAA 10 has a modemchip set or modem 40, an optically-coupled transhybrid circuit 50, ahook switch circuit 60, and a ring detection circuit 70.

The modem 40 converts the analog signals on the telephone line to adigital format compatible with the subscriber's equipment. The modem 40has four ports: transmit 100, receive 102, hook switch control output104, and ring detection input 106.

The optically-coupled transhybrid circuit 50 described here providesisolation between the telephone company network 20 and the subscriber'sdigital bus 30, and separates the transmit and receive signal paths. Thetranshybrid 50 has four ports: a transmit input port 120, a receiveoutput port 122, and tip and ring line ports 130 and 132. Thetranshybrid 50 contains isolated transmit and receive networks thatprovide the return path for the central office loop current and the2-to-4 wire separation function, and isolate the network 20 from thesubscriber bus 30.

Signals originating in the subscriber equipment 32 (modem transmitsignals) pass through the modem 40, exiting at the modem's transmit port100, into the transhybrid 50 at the transmit input port 120, and exit atthe tip and ring line ports 130 and 132, and onto the tip and ring lines22 and 24.

Signals received from the telephone network 20 (i.e., from the centraloffice) enter the transhybrid 50 at the tip and ring line ports 130 and132, pass through the transhybrid 50, and exit the transhybrid 50 at thereceive output port 122. The signals then pass into the modem 40 by wayof the receive port 102.

The hook switch circuit 60 presents a D.C. continuity to the telephonenetwork 20 when a call is in progress. The circuit 60 sits in the ringline 24 of the telephone network 20, connecting by line ports 160 and162. It receives triggering from the modem 40 at its modem input 164.

Although such circuits have typically used an electromechanical relay tomake the actual connection to the transhybrid 50, an optically-coupledFET solid state relay will perform the same function. The hook switchcircuit 60 in effect connects the subscriber equipment 32 to thetelephone network 20 when the subscriber places a call or the ringdetection circuit 70 senses an incoming call, causing the subscriberequipment 32 to answer the call.

The ring detection circuit 70 straddles the tip and ring lines 22 and 24of the telephone network 20, connecting to those lines at tip and ringline ports 170 and 172, respectively. One of the functions of the ringdetection circuit 70 is to provide a signal to the subscriber equipment32, indicating the presence of an incoming call. Here, the ringindication passes from a ring indication output 174 to the ringdetection input 106 of the modem 40.

The Optically-Coupled Transhybrid

The optically-coupled transhybrid is shown in FIG. 2. The transhybrid 50serves a dual function: it separates the modem transmit and receivesignal paths and it provides isolation between the telephone network 20and the subscriber equipment 32. The separation is achieved bysuppressing the level of the modem transmit signal at the transhybrid'soutput 122 to the modem receive input 102.

The transhybrid 50 has three active elements: a first isolationamplifier U101, a second isolation amplifier U102, and a transmitsuppression and receive amplifier U103. So that it will function as adifference amplifier, amplifier U103 is provided with a feedbackresistor R. In keeping with the telephone network requirement thattelephone signals remain at constant average amplitude, each amplifierhas unity gain. A dashed line running through U101 and U102 delineatesan isolation barrier between the subscriber side and the telephonenetwork side of the transhybrid 50.

As shown in FIG. 2, the tip and ring lines 22 and 24 are connectedacross a diode bridge D101-D104 and a symbolic load resistor R_(L). Anincoming ("received") signal from the telephone network 20 enters thetranshybrid 50 through the diode bridge D101-D104 and is coupled throughcapacitor C to the input of the second isolation amplifier U102.

The output of amplifier U102 is coupled to the non-inverting input ofthe transmit suppression and receive amplifier U103. The output voltageof amplifier U103 is equal to the difference between the voltages at thenon-inverting and inverting inputs of the amplifier U103, multiplied bythe open-loop gain of the amplifier. Since a signal is being receivedand the modem is not transmitting, no signal is present at the invertinginput of amplifier U103. Thus, the received signal passes throughamplifier U103 to the receive output port 122.

In the case of an outbound or modem transmit signal, the transmit inputport 120 of the transhybrid 50 receives the output of the modem andpasses it to the input of a first isolation amplifier U101 having unitygain. The output of the first isolation amplifier U101 is connected tothe diode bridge D101-D104, which passes the modem transmit signal tothe telephone network tip and ring lines and 24.

In addition to reaching the telephone network 20, the modem transmitsignal also reaches both inputs of the transmit suppression and receiveamplifier U103. It passes through capacitor C and second isolationamplifier U102 to the non-inverting input of amplifier U103. As a resultof passing through amplifiers U101 and U102 and capacitor C, the signalarriving at the non-inverting input experiences group delay andfrequency response distortion. For that reason, the signal is alsopassed through a group delay equalizer 240 to the inverting input ofamplifier U103. The group delay equalizer 240 compensates for thesignal's passage through amplifiers U101 and U102 and capacitor C byproviding an equal delay and frequency response alteration such that themodem transmit signal at the inverting input of amplifier U103 is inphase with the signal appearing at the non-inverting input. Since thesignals are equal in amplitude and in phase, they cancel or, in otherwords, the signal issuing from the group delay equalizer 240 suppressesthe signal reaching the non-inverting input of the amplifier U103.

The effectiveness of the transhybrid 50 is determined by the followingrelationship: ##EQU1##

where V_(T) is the amplitude of the signal at the transmit input port120, and

V_(R) is the amplitude of the signal at the receive output port 122.

When V_(T) is much greater than V_(R), there is high transhybrid loss,or rejection, which is desirable.

An Implementation of the Optically Coupled Transhybrid in a DAA

FIG. 3 is a detailed schematic of the optically-coupled transhybrid 50,the hook switch circuit 60, and the ring detection circuit 70. A dashedline denotes the intersection of the isolated subscriber equipment(labelled "subscriber side" in the figure) and telephone networks(called the "telco line side" in the figure). The explanation of thesystem of FIG. 3 will begin with a transmit signal from the subscriberequipment 32, followed by a discussion of the transfer of a signalreceived on the telephone network 20 to the subscriber equipment 32.

The modem transmit signal enters the transhybrid 50 through a couplingcapacitor C1 and a resistor R7, passing to the inverting input of adifferential amplifier U1b. The amplifier U1b, together with an opticalcoupler U2, a transistor Q1, a second differential amplifier U3a, and asecond transistor Q2, function as the first isolation amplifier U101 ofFIG. 2. The modem transmit signal is also applied to a group delayequalizer 240, consisting of resistors R1, R2, R3 and capacitors C2 andC3, as shown in the schematic of FIG. 3.

Amplifier U1b, transistor Q1, optical coupler U2, differential amplifierU3a, and transistor Q2 are collectively configured to exhibit unitygain. The inverting input of amplifier U1b is biased by a network ofresistors R8 and R9 and capacitor C4.

The signal passes through differential amplifier U1b and is supplied tooptical coupler U2 by transistor Q1 connected as an emitter-followeramplifier. Resistors R12 and R13 decrease the open-loop gain oftransistor Q1 by limiting circuit bandwidth and current, thus providingoverall stability. Coupler U2 has a feedback photodiode P1 connected tocoupler terminals 3 and 4; a "rail-splitter" network of resistors R10and R11 and capacitor C5 supplies the feedback photocurrent for thephotodiode P1 and is connected to the non-inverting input of amplifierU1b.

The signal appearing at the output of coupler U2 at coupler terminals 5and 6 is connected through a resistor network R14-R15-R16-R17 to thenon-inverting input of amplifier U3a. A resistor R18 connected to theinverting input of the amplifier U3a controls the gain of the amplifier.The combination of a resistor R20, a capacitor C7, and a zener diode Z1functions as a power supply for amplifier U3a.

The signal at the output of amplifier U3a is coupled through a resistorR19 to transistor Q2. The output of Q2 is coupled to a diode bridgeD3-D6, which in turn is connected to the tip and ring lines 22 and 24,completing the path from the transmit input port 120 to the telephonenetwork 20.

The transmit signal present at the output of transistor Q2 will alsoattempt to reach the receive output port 122 of this circuit. It passesthrough a coupling capacitor C9 and a resistor R21 to the invertinginput of a third differential amplifier U3b. A pair of back-to-backdiodes D1-D2 across the inverting and non-inverting inputs of amplifierU3b protects the amplifier from excessive voltages. The amplifier U3b isbiased at its non-inverting input by a network of resistors R22 and R23and capacitor C10.

The signal at the output of amplifier U3b is coupled to a transistor Q3.As with transistor Q1, resistors R26 and R27 decrease the open-loop gainof transistor Q3 by limiting circuit bandwidth and current, thusproviding overall stability for that transistor.

Configured as an emitter-follower amplifier, transistor Q3 drives theLED in an optical coupler U4. Coupler U4 has a feedback photodiode P1connected to coupler terminals 3 and 4; a "rail-splitter" network ofresistors R24 and R25 and capacitor C11 supplies the feedbackphotocurrent for the photodiode P1. The feedback is appropriatelysupplied to the non-inverting input of amplifier U3b.

Amplifier U3b, transistor Q3, and coupler U4 function collectively asthe second isolation amplifier U102 of FIG. 2. As with the componentscomprising amplifier U101, the gain through this path is also unity.

The output of optical coupler U4 is a current that develops a voltageacross resistor R28. This voltage is coupled through a resistor R4 tothe non-inverting input of a transmit suppression and receive amplifierU1a, represented by amplifier U103 in FIG. 2. Simultaneously, the samesignal that originated at the transmit input port 120 passes through thegroup delay equalizer 240 to the non-inverting input of amplifier U1a. Afeedback resistor R5 is connected between the amplifier's non-invertinginput and output while a resistor R6 provides bias from the supplyvoltage to the non-inverting input of amplifier U1a.

The equalizer 240 assures that the transmit signals appearing at theinverting and non-inverting inputs of the transmit suppression andreceive amplifier U1a arrive in-phase with respect to each other. Sincethe voltages at the two inputs are also of the same amplitude, the twosignals effectively cancel and the net output of amplifier U1a is zero.

Receiving a Signal on the Telephone Network

If a signal is received on the telephone network 20, it enters throughthe diode bridge D3-D6, proceeds through capacitor C9 to amplifier U3b,transistor Q3, and optical coupler U4, and into the non-inverting inputof amplifier U1a. Because the received signal does not reach theinverting input of amplifier U1a, the received signal effectively passesthrough amplifier U1a to the receive output port 122. Thus, theseparation of the transmit and receive paths is accomplished along withan isolation of the telephone network 20 from the subscriber equipment32.

Each of the amplifiers U1b, U3a, and U3b have respective feedbackcapacitors C5, C8, and C12 to control the roll-off response of theamplifiers.

The hook switch circuit 60 can be implemented with acommercially-available optical Coupler U5. The hook switch circuit 60sits in the ring line 24, connected at coupler terminals 6 and 8. Thecontrol signal for the hook switch circuit 60 is received from the modem40 (FIG. 1) through a resistor R31.

The ring detection circuit 70 can also be implemented with acommercially-available optical coupler U6. As shown, the ring detectioncircuit 70 straddles the tip and ring lines 22 and 24. A couplingcapacitor C13 and zener diode Z2 connect to coupler terminal 2 of thecoupler U6 while a resistor R30 connects to coupler terminal 3 of thecoupler U6. A pair of diodes D7-D8 in series across coupler terminals 2and 3 will discharge any voltage buildup on the coupling capacitor C13.With aid of a pull-up resistor R29, the output of the circuit issupplied at coupler terminal 7.

The hook switch circuit 60 and the ring detection circuit 70 areconnected to a subscriber isolated ground GND1; this ground is isolatedfrom the isolated ground GND2 grounding the diode bridge D3-D6. Thisalso implies isolated power supplies--one supply Vcc₁ is on thesubscriber side; another V_(CC2) is on the telephone network side. Forprotection at the telephone network 20 input to the DAA 10, ametal-oxide varistor MOV can be placed across the tip and ring lines 22and 24.

The devices described here are based in part on the technology used inthe Siemens®IL300 family of aluminum gallium arsenide (AlGaAs) linearoptocouplers, discussed in the Siemens Optoelectronics Data Book 1993,pp. 5-115 through 5-122, and pp. 11-177 through 11-193. It should beunderstood that other devices can be used.

Instead of the specific implementation shown in FIG. 3, one couldimplement the DAA and the optically-coupled transhybrid using otherdiscrete and/or integrated solutions such as hybrid microcircuits. Also,the DAA and transhybrid can be implemented with digital signalprocessing (DSP) techniques, representing the separation/suppression andisolation functions as an algorithm (or a series of algorithms).

While there has been described what is believed to be the preferredembodiment of the invention, those skilled in the art will recognizethat other and further modifications may be made thereto withoutdeparting from the spirit of the invention, and it is intended to claimall such embodiments that fall within the true scope of the invention.

What is claimed is:
 1. A data access arrangement for interconnecting atelephone network of tip and ring lines and a subscriber digital databus, comprising:a modem for bidirectionally converting analog signals todigital signals, having a bidirectional digital bus port for connectionto the subscriber digital data bus, an analog transmit port, and ananalog receive port; and means for connecting the modem to the telephonenetwork and isolating the modem from the telephone network, the meansfor connecting and isolating havingfirst means for transferring signalsreceived on the tip and ring lines of the telephone network to theanalog receive port of the modem and suppressing signals originating atthe analog transmit port of the modem, the first means having an outputprovided to the analog receive port of the modem; second means forisolating the tip and ring lines of the telephone network from the modemand transferring signals originating at the analog transmit port of themodem to the tip and ring lines of the telephone network, the secondmeans being connected between the analog transmit port of the modem andthe tip and ring lines of the telephone network; and third means forisolating the tip and ring lines of the telephone network from the modemand transferring signals received on the tip and ring lines of thetelephone network, the third means being connected between the tip andring lines of the telephone network and the first means.
 2. Theapparatus as set forth in claim 1, wherein the second and third meansare optical isolators.
 3. The apparatus as set forth in claim 1, whereinthe first means includes a differential amplifier havinga first inputconnected to the third means; a second input responsive to the analogtransmit port of the modem; and an output provided to the output of theanalog receive port of the modem.
 4. The apparatus as set forth in claim3, wherein the first means further includes equalizing means forcontrolling group delay and amplitude with respect to frequency, theequalizing means being connected between the analog transmit port of themodem and the second input of the differential amplifier.
 5. Acommunications apparatus for transmitting and receiving signals on atelephone network of tip and ring lines, comprising:a subscriber device,the subscriber device having a port for transmitting and receivingsignals on a digital data bus; and a data access arrangement forinterconnecting the telephone network of tip and ring lines and thesubscriber device, the data access arrangement having:a modem forbidirectionally converting analog signals to digital signals, having abidirectional digital bus port for connection to the digital data bus,an analog transmit port, and an analog receive port; and a transhybridhavinga transmit suppression and receive amplifier having a first input,a second input responsive to signals originating at the analog transmitport of the modem, and an output provided to the analog receive port ofthe modem; a first optical isolator connected between the analogtransmit port of the modem and the tip and ring lines of the telephonenetwork; and a second optical isolator connected between the tip andring lines of the telephone network and the first input of the transmitsuppression and receive amplifier.
 6. The apparatus as set forth inclaim 5, further including a group delay equalizer connected between theanalog transmit port of the modem and the second input of the transmitsuppression and receive amplifier.